Binary spatial light modulators are typically comprised of an array of elements each having two states, on and off. The use of pulse width modulation (PWM) is one conventional approach of digitally displaying incoming analog video data, as compared to an analog display such as a cathode ray tube (CRT) based system. PWM typically comprises dividing a frame of incoming video data into weighted segments. For example, for a system that samples the luminance component of incoming video data in 8-bit samples, the video frame time is divided up into 255 time segments or pixel values (28−1). Conventionally, the 8-bit samples are formatted with binary values. The most significant bit (MSB) data is displayed on a given element for 128 time segments. In the present example, the next MSB has a time period of 64 time segments, and so on, such that the next bits have weights of 32, 16, 8, 4, 2 and 1 time segments, consecutively. Thus, the least significant bit (LSB) has only one time segment. All pixel values are comprised of a summation of these weighted bits.
In DMD display systems, such as disclosed in commonly assigned U.S. Pat. No. 5,278,652 entitled “DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System”, the teachings of which are incorporated herein by reference, light intensity for each pixel is typically displayed as a linear function of the pixel digital codes. For an 8-bit binary code, 0 is no light, 255 is peak light, and 128 is midscale light. Codes between 0 and 255 form a grayscale in each color. This grayscale sets the image resolution for the system by defining the number of discrete levels of light that can be produced for each color; i.e. red, green and blue. Pulse width modulation (PWM) schemes used to control the mirrors conventionally modulate the mirrors using bit-planes having weights based on powers of two. For example, 20 μs, 40 μs, 80 μs, 160 μs, 320 μs, 640 μs, 1280 μs, and 2,560 μs are used to define the mirror on-times for the 8 bit-planes needed for 8-bit video where 5.5 ms is available per color. Light is transmitted to the display screen as black for the bit-plane of a pixel which is logic 0 or at full brightness during a bit-plane which is logic 1. Since the on-times for bit-planes vary, this results in PWM over a frame period. The viewer's eyes integrate the modulated light so that gray levels are formed and perceived.
A problem arises when using the PWM technique because the light is displayed in series of discrete bursts during each frame. The shifts in ordering of these discrete bursts, as the displayed graycodes vary, generate artifacts in some images. For adjacent pixels, where major bit transitions take place, the sudden change in the ordering (and therefore time phase) of the discrete light burst within a frame causes noticeable pulsations in images upon viewing. Viewer's eyes integrate the out of phase ordering of mirror modulation, for adjacent pixels, to create the pulsations. These pulsations are referred to as PWM temporal contouring (hereafter referred to as simply PWM contouring), shown in FIG. 1, because they create apparent contours in images that are time-varying. In commonly assigned U.S. Pat. No. 5,619,228 entitled “Method for Reducing Temporal Artifacts in Digital Video Systems”, there is disclosed one method of mitigating PWM contouring, the teachings of which are incorporated herein by reference.
PWM contouring can most clearly be seen on a grayscale ramp that goes horizontally across the screen. Here, vertical pulsations are seen at many major bit transitions when a viewer's eyes are scanned horizontally across the screen. When a viewer's eyes scan, the eyes integrate light only briefly over any given part of the screen. The viewer's scanning eyes catch the transmitted light for adjacent pixels out of time phase and pulsations are seen on the screen.
At normal viewing distance, PWM contouring for two adjacent pixels is difficult or impossible to resolve. However, in real images, boundary conditions often exists where many pixels are spatially bunched together with codes near each other (a sky scene for example). If these codes have clusters that cross a major bit transition, while others don't, PWM contouring will occur.
It is desirable to display data on a digital display, such as a DMD, with reduced PWM contouring artifacts without increasing system bandwidth.